To feel or see the difference between aspirated and unaspirated sounds, one can put a hand or a lit candle in front of one's mouth, and say spin[spɪn] and then pin[pʰɪn]. One should either feel a puff of air or see a flicker of the candle flame with pin that one does not get with spin.

There are no dedicated IPA symbols for degrees of aspiration and typically only two degrees are marked: unaspirated ⟨k⟩ and aspirated ⟨kʰ⟩. An old symbol for light aspiration was ⟨ʻ⟩, but this is now obsolete. The aspiration modifier letter may be doubled to indicate especially strong or long aspiration. Hence, the two degrees of aspiration in Korean stops are sometimes transcribed ⟨kʰ kʰʰ⟩ or ⟨kʻ⟩ and ⟨kʰ⟩, but they are usually transcribed [k] and [kʰ],[1] with the details of voice onset time given numerically.

Preaspirated consonants are marked by placing the aspiration modifier letter before the consonant symbol: ⟨ʰp⟩ represents the preaspirated bilabial stop.

Unaspirated or tenuis consonants are occasionally marked with the modifier letter for unaspiration ⟨◌˭⟩, a superscriptequals sign: ⟨t˭⟩. Usually, however, unaspirated consonants are left unmarked: ⟨t⟩.

Voiceless consonants are produced with the vocal folds open (spread) and not vibrating, and voiced consonants are produced when the vocal folds are fractionally closed and vibrating (modal voice). Voiceless aspiration occurs when the vocal folds remain open after a consonant is released. An easy way to measure this is by noting the consonant's voice onset time, as the voicing of a following vowel cannot begin until the vocal folds close.

Phonetically in some languages, such as Navajo, aspiration of stops tends to be realised as voiceless velar airflow; aspiration of affricates is realised as an extended length of the frication.

Aspirated consonants are not always followed by vowels or other voiced sounds. For example, in Eastern Armenian, aspiration is contrastive even word-finally, and aspirated consonants occur in consonant clusters. In Wahgi, consonants are aspirated only in final position.

The degree of aspiration varies: the voice onset time of aspirated stops is longer or shorter depending on the language or the place of articulation.

Armenian and Cantonese have aspiration that lasts about as long as English aspirated stops, in addition to unaspirated stops. Korean has lightly aspirated stops that fall between the Armenian and Cantonese unaspirated and aspirated stops as well as strongly aspirated stops whose aspiration lasts longer than that of Armenian or Cantonese. (See voice onset time.)

Aspiration varies with place of articulation. The Spanish voiceless stops /p t k/ have voice onset times (VOTs) of about 5, 10, and 30 milliseconds, whereas English aspirated /p t k/ have VOTs of about 60, 70, and 80 ms. Voice onset time in Korean has been measured at 20, 25, and 50 ms for /p t k/ and 90, 95, and 125 for /pʰ tʰ kʰ/.[2]

When aspirated consonants are doubled or geminated, the stop is held longer and then has an aspirated release. An aspirated affricate consists of a stop, fricative, and aspirated release. A doubled aspirated affricate has a longer hold in the stop portion and then has a release consisting of the fricative and aspiration.

Although most aspirated obstruents in the world's languages are stops and affricates, aspirated fricatives such as [sʰ], [ɸʷʰ] or [ɕʰ] have been documented in Korean, though these are allophones of other phonemes. Similarly, aspirated fricatives and even aspirated nasals, approximants, and trills occur in a few Tibeto-Burman languages, in some Oto-Manguean languages, in the Hmongic language Hmu, and in the Siouan language Ofo. Some languages, such as Choni Tibetan, have as many as four contrastive aspirated fricatives [sʰ][ɕʰ], [ʂʰ] and [xʰ].[3]

True aspirated voiced consonants, as opposed to murmured (breathy-voice) consonants such as the [bʱ], [dʱ], [ɡʱ] that are common in the languages of India, are extremely rare. They have been documented in Kelabit[4]Taa, and the Kx'a languages. Reported aspirated voiced stops, affricates and clicks are prevoiced[b͡pʰ, d͡tʰ, d͡tsʰ, d͡tʃʰ, ɡ͡kʰ, ɢ͡qʰ, ᶢᵏʘʰ, ᶢᵏǀʰ, ᶢǁʰ, ᶢᵏǃʰ, ᶢᵏǂʰ].[citation needed]

They are unaspirated for almost all speakers when immediately following word-initial s, as in spill, still, skill. After an s elsewhere in a word they are normally unaspirated as well, except sometimes in compound words. When the consonants in a cluster like st are analyzed as belonging to different morphemes (heteromorphemic) the stop is aspirated, but when they are analyzed as belonging to one morpheme the stop is unaspirated.[citation needed] For instance, distend has unaspirated [t] since it is not analyzed as two morphemes, but distaste has an aspirated middle [tʰ] because it is analyzed as dis- + taste and the word taste has an aspirated initial t.

Word-final voiceless stops are sometimes aspirated.

Voiceless stops in Pashto are slightly aspirated prevocalically in a stressed syllable.

In Danish and most southern varieties of German, the lenis consonants transcribed for historical reasons as ⟨b d ɡ⟩ are distinguished from their fortis counterparts ⟨p t k⟩, mainly in their lack of aspiration.[citation needed]

Wu Chinese and Southern Min has a three-way distinction in stops and affricates: /p pʰ b/. In addition to aspirated and unaspirated consonants, there is a series of muddy consonants, like /b/. These are pronounced with slack or breathy voice: that is, they are weakly voiced. Muddy consonants as initial cause a syllable to be pronounced with low pitch or light (陽 yáng) tone.

Many Indo-Aryan languages have aspirated stops. Sanskrit, Hindi, Bengali, Marathi, and Gujarati have a four-way distinction in stops: voiceless, aspirated, voiced, and breathy-voiced or voiced aspirated, such as /p pʰ b bʱ/. Punjabi has lost breathy-voiced consonants, which resulted in a tone system, and therefore has a distinction between voiceless, aspirated, and voiced: /p pʰ b/.

Western Armenian has a two-way distinction between aspirated and voiced: /tʰ d/. Western Armenian aspirated /tʰ/ corresponds to Eastern Armenian aspirated /tʰ/ and voiced /d/, and Western voiced /d/ corresponds to Eastern voiceless /t/.

There were aspirated stops at three places of articulation: labial, coronal, and velar /pʰ tʰ kʰ/. Earlier Greek, represented by Mycenaean Greek, likely had a labialized velar aspirated stop /kʷʰ/, which later became labial, coronal, or velar depending on dialect and phonetic environment.

The other Ancient Greek dialects, Ionic, Doric, Aeolic, and Arcadocypriot, likely had the same three-way distinction at one point, but Doric seems to have had a fricative in place of /tʰ/ in the Classical period, and the Ionic and Aeolic dialects sometimes lost aspiration (psilosis).

Later, during the Koine Greek period, the aspirated and voiced stops /tʰ d/ of Attic Greek lenited to voiceless and voiced fricatives, yielding /θ ð/ in Medieval and Modern Greek.

So-called voiced aspirated consonants are nearly always pronounced instead with breathy voice, a type of phonation or vibration of the vocal folds. The modifier letter ⟨◌ʰ⟩ after a voiced consonant actually represents a breathy-voiced or murmured dental stop, as with the "voiced aspirated" bilabial stop ⟨bʰ⟩ in the Indo-Aryan languages. This consonant is therefore more accurately transcribed as ⟨b̤⟩, with the diacritic for breathy voice, or with the modifier letter ⟨bʱ⟩, a superscript form of the symbol for the voiced glottal fricative ⟨ɦ⟩.

Some linguists restrict the double-dot subscript ⟨◌̤⟩ to murmured sonorants, such as vowels and nasals, which are murmured throughout their duration, and use the superscript hook-aitch ⟨◌ʱ⟩ for the breathy-voiced release of obstruents.